Abstract
A comprehensive petrophysical study of the Upper Qishn Clastic Member reservoir was conducted at Tawila oilfield in Yemen. An integrated well-logging dataset was analyzed to detect the reservoir properties and hydrocarbon potential. Lithologic data, shale/matrix models, elastic, seismo-facies, and fluid analytical analyses and crossplots were generated to characterize the reservoir. The clastic reservoir was subdivided into four petrophysical zones (S1A, S1B, S2, and S3) having different reservoir properties and hydrocarbon content. Crossplots of dia-porosity, apparent-matrix density, and elastic rock properties revealed a reservoir with mixed rock types primarily composed of quartz, calcite, and feldspars, with rare kaolinite and heavy minerals. A laminated-shale model was found to apply to the S2 and S3 reservoir zones, while dispersed- and structural-shale models best describe the S1A and S1B zones. Pickett and Buckles saturation crossplots indicated that the reservoir is not at irreducible water saturation. The Qishn reservoir exhibits regular and uniform facies of similar thickness throughout the study area with a slight dip toward the northwestern part of the oilfield. Estimated petrophysical parameters for the reservoir are represented by vertical petrophysical analogs and property distribution maps. The vertical petrophysical analogs show that the S2 and S3 zones attained clearly defined reservoir properties compared to S1A and S1B zones. The S2 zone has an effective porosity of 11-19%, permeability of 2-670 mD, and hydrocarbon saturation < 57%. The S3 zone offers an effective porosity of 12-21%, permeability of 4-2000 mD, and hydrocarbon saturation < 61%. Both S1A and S1B zones show weak petrophysical parameters and low hydrocarbon content. Hydrocarbon-distribution maps reveal significant hydrocarbon saturation north of the main east-west-trending fault reaching a maximum associated with low shale volume and excellent effective porosity and permeability values. It was concluded that the S2 and S3 zones are the most hydrocarbon-rich zones in the reservoir and should be further studied and mapped across the study area and the nearby oilfields.